1. Field of the Invention
The present invention relates to a chirp measurement apparatus and its application for detecting wavelength dispersion and its fluctuations in an optical fiber in a terminal or in a linear repeater and regenerator of a ultrafast, large capacity optical communication system. In addition, the present invention relates to a chirp measurement method for carrying out calibration using input signal light unaffected by the wavelength dispersion of the optical fiber.
2. Description of the Related Art
Optical fiber transmission lines have dispersion fluctuations due to environmental changes such temperature variations and pressure application. Accordingly, it is necessary for ultrafast large capacity optical communication systems to employ an adaptive dispersion equalization technique for automatically detecting the dispersion fluctuations of the optical fiber transmission lines to carry out equalization. As conventional chirp measurement methods for automatically detecting the dispersion fluctuations, the following methods are known.
An adaptive dispersion equalization scheme utilizing a wavelength tunable laser: It monitors the intensity of a 40 GHz electric clock, and dithers the wavelength of the signal light to set it at the optimum wavelength (G. Ishikawa et al., ECOC'98, p.519, 1998). It has a problem in that the wavelength of the signal light varies.
An adaptive dispersion equalization scheme utilizing a VIPA (Virtually Imaged Phased Array) tunable dispersion equalizer: It employs a method of dithering the dispersion of an equalizer to detect the fluctuations in the dispersion (H. Ooi et al., OECC'2001, PD5, 2001). It dithers the dispersion value in a range of ±3 ps/nm, and has a problem in that it cannot avoid characteristic degradation in the long run because it includes an movable optical section.
An adaptive dispersion equalizer using a fiber grating: It employs a method of detecting the dispersion fluctuations in a single mode fiber transmission line by PM-AM conversion (K. M. Feng et al., IEEE Photon. Technol. Lett., vol.11, no.3, p.373, 1999). The single mode fiber always has anomalous dispersion in 1.5 μm band, and can utilize the PM-AM conversion because the sign of the dispersion is invariant against the environmental change. However, as for a dispersion shift fiber transmission line, the sign of the dispersion of which varies, a monitor signal is required besides a main signal (M. Tomizawa et al., J. Lightwave Technol., vol.16, no.2, p.184, 1998).
There is another method that detects the dispersion fluctuations by measuring the relative phase shift amount between two channels of the WDM signal (A. Sano et al, OFC'99, WJ4, p.165, 1999). This method has a problem of requiring a monitor signal in addition to the main signal.
Furthermore, a configuration as shown in FIG. 1 is known (Japanese Patent Application Laid-open No. 2001-053679, and T. Inui et. al., IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.14, No.4, April 2002). It splits the signal light affected by the dispersion fluctuations through an optical fiber transmission line into two portions, supplies them to optical fibers which are arranged in two paths and have the dispersion values having the same absolute value and different sign, and detects the dispersion fluctuations from the difference between the levels of the clock signals of the two signal light beams passing through the two paths.
In FIG. 1, a first optical coupler divides part of the signal light from the optical fiber transmission line, and a second optical coupler splits it to two portions to a path 1 and path 2. An optical fiber with positive dispersion (+D ps/nm) is disposed as the path 1, and an optical fiber with negative dispersion (−D ps/nm) is disposed as the path 2. The signal light beams passing through the optical fibers constituting the two paths are each converted into an electric signal by a photodiode (PD), and supplied to an RF detector for detecting the level of the clock signal via a bandpass filter (BPF). The levels of the clock signals of the two paths are compared by a differential amplifier, which outputs a differential signal.
The optical fiber transmission line has dispersion fluctuations due to environmental changes, and the dispersion fluctuations make the level of the clock signal (output voltage) of one of the two paths greater than that of the other path as illustrated in FIG. 2A. In this case, controlling a tunable dispersion equalizer (not shown) inserted into the optical fiber transmission line can minimize the differential signal so that the levels of the clock signals of the two paths become equal as illustrated in FIG. 2B, thereby being able to equalize the dispersion fluctuations at high accuracy.
Thus, the conventional configuration as shown in FIG. 1 can detect the dispersion fluctuations with the fluctuation direction of the dispersion of the optical fiber transmission line by using the differential signal between the levels of the clock signals of the two paths. Accordingly, it is applicable to various types of optical fiber transmission lines.
However, the chirp measurement apparatus with the conventional configuration must determine the passband of its clock extraction circuit (bandpass filter and RF detector) in accordance with the bit rate of the transmission system. This is because it establishes synchronization by extracting the clock signal. As a result, a new problem arises that it is difficult for the conventional system to flexibly cope with considerable changes in the bit rate of the optical signal to be measured.